Arsenic concentrates in the kidneys of the giant clams of Australia's Great Barrier Reef. The highest concentrations measured were 1004 parts per million, of which most, 2066 parts per million, were in the water-soluble fraction containing trimethylarsoniumlactate and its derivatives. This accumulation is ascribed to a mechanism in which oceanic arsenate is assimilated by symbiotic zooxanthellae and subsequently deposited in host tissues. The gills are the major site of arsenic excretion by these animals. Gill membrane arsenolipids mediate exposure of their trimethylarsonium groups to the sea and its biological oxidative activities.